The objective of this proposal is to enhance understanding of normal bone development and its determinants by extending our assessments of the unique IBDS cohort into early adulthood (age 23 years) to address major questions about the important transition period from late adolescence to early adulthood. We will obtain age 23 bone outcomes (DXA, pQCT, and MDCT); collect detailed dietary, alcohol, tobacco, sleep, physical activity and body composition data; and model the variability in individual development of whole bone, and trabecular and cortical bone micro-architecture. Specifically, we propose to: 1) Determine the impact of changes in physical attributes (weight, adipose tissue, lean body mass), and lifestyle behaviors (physical activity, dietary intakes, tobacco use, and sleep) during the adolescent-adult transition (measured at 19, 21 and 23 years of age) on changes in measures of bone mass, density, geometry and micro-architecture derived from DXA, pQCT, and MDCT (measured at 19 and 23 years of age), and 2) Assess the relationships between levels and patterns of physical attributes, physical activity, dietary intakes, and other factors from 5 to 23 years of age and during the pre-pubertal, peri-pubertal, and post-pubertal developmental periods on measures of bone mass, density, geometry and micro-architecture, assessed both at age 23 when bone development is largely complete, as well as longitudinally from age 5 to 23. Late adolescence/early adulthood is a period of major lifestyle behavior changes, but major gaps remain in our understanding of the effects of modifiable and non-modifiable factors on bone development during the adolescent-adult transition period. This period is important in determining future risk for osteoporosis and bone fractures. Few studies have assessed bone outcomes and their determinants longitudinally from middle childhood to early adulthood. The Iowa Bone Development Study (IBDS), in its 17th year, is investigating the effects of physical attributes, lifestyle behaviors, dietary intakes, and other factors on bone development and micro-architecture. We assessed bone measures at ages 5, 8, 11, 13, 15, and 17 years using dual-energy x-ray absorptiometry (DXA) and peripheral quantitative computed tomography (pQCT), collected dietary data from birth, and assessed physical activity by accelerometry and questionnaire from age 5. Analogous cross-sectional measures for cohort parents are also available. Age 19 exams are in progress (n=255 to date), with the addition of high-resolution multi-detector CT scans (MDCT) of the distal tibia and advanced algorithms to assess bone micro-architecture, advanced 3-D accelerometry, and more detailed lifestyle questionnaires. Continued follow-up of the IBDS cohort to early adulthood with emphasis on micro-architecture will provide major insights into the relative importance of the many explanatory factors on bone development and strength and the developmental period(s) of greatest impact, providing a strong foundation for future design of strategies and interventions to optimize skeletal health.